1. Field of the Invention
The present invention relates to a blade tip clearance control apparatus for use with a gas turbine engine. In particular the invention is concerned with providing a clearance control apparatus for a gas turbine engine to control the clearance between a casing or static portion of the engine and the tips of turbine blades.
2. Description of the Related Art
It is important to keep the clearance between the tips of rotating blades and a static portion, such as the radially inner surface of an annular shroud, which surrounds the blade tips to a minimum. The clearance is controlled to minimize the leakage of turbine gases between the shroud and the blade tips. Minimizing the leakage of the turbine gases improves the engine efficiency and thereby reduces the specific fuel consumption of the engine.
During the conventional operating cycle of a gas turbine engine the turbine blades, and the discs on which they are mounted, expand due to centrifugal forces acting on them as they rotate at high speeds and by thermal expansion due to being heated by the working fluid passing therethrough. The annular shroud however is stationary and only expands due to being heated by the working fluid. Differential expansion occurs and the clearance between the blade tips and the shroud has to be controlled to give a minimum clearance at steady state conditions whilst ensuring that the blade tips do not rub on the shroud during transients.